Why Change?

The weather has taken a sudden turn here in the midwest. A dramatic drop in temperatures and rainy, stormy weather have us all thinking about the upcoming fall. And with fall comes the annual changing of the guard so to speak. The broadleaf trees of Michigan undergo a spectacular change of color which has supported fall color tours and a booming tourism industry. Yet have you ever wondered what causes the leaves to change in the first place? And why does the color seem brighter and more vibrant than in others? Read on to find out!

A Reason to Change

By: Jennifer Hunnell, Education/Outreach Program Coordinator, Eaton Conservation District

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There are many reasons to enjoy the fall season – the fruit harvests, the cooler weather, the fun outdoor activities such as corn mazes, hayrides and pumpkin picking, and of course the beautiful fall colors. Every year across the country thousands travel to their favorite spots to see the best fall foliage. But why do the leaves even change?

It’s all about nutrient recovery. Leaves are the tree’s powerhouse. This is where the conversion of sunlight occurs, generating food for the tree through photosynthesis, and allowing it to grow and reproduce. The driving force behind photosynthesis is a compound called chlorophyll. And chlorophyll is what gives leaves their green appearance. Contrary to popular belief, the temperature outside has nothing to do with determining when the leaves change. It is dictated primarily by day length. No other environmental influence is as consistent as the increasing length of night during autumn. This acts as a signal for plants to start preparing for winter, initiating the pull-back of nutrients from the leaves. The production of chlorophyll slows down, eventually stopping completely. This breakdown of the chlorophyll reveals other compounds already within the leaves that show through as orange, yellow, red and brown. So the beautiful colors we associate with fall were there all along; hidden until the time was right.

An interesting thing about color change is that the color quality in a particular year is weather-dependent. The amount of yellows and oranges from year to year remains fairly constant, as the amounts of the compounds creating them stay much the same. Reds, however, are another story. A succession of warm, sunny days followed by cool, but not freezing nights results in the production of a lot of anthocyanins. Anthocyanin is the compound responsible for hues of red and purple. So years with this combination of fall weather produce vibrant red colors, especially in sugar maples. The amount of rain in a year matters too. A severe drought can delay leaf turn, whereas a severe frost will kill leaves, causing them to drop early. A warm, wet period during fall lowers the brightness of the colors, but a warm, wet spring favors fall foliage. Confused yet? The bottom line is there are several factors that go into the brilliance of autumn leaf colors. Here’s the basic formula for outstanding fall color:

  • A warm, wet spring
  • A summer that’s not too hot or dry, and
  • A fall with plenty of warm sunny days and cool nights

So if you see a lot of brilliant, beautiful colors this fall, credit the weather.

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